1. FIELD OF THE INVENTION
This invention relates to a selectively releasable and re-engagable expansion joint for use in connecting a tubing string to a packer set at a preselected location in a subterranean well.
2. SUMMARY OF THE PRIOR ART
In the treatment and production of subterranean oil and gas wells, it has been common to run a tubing string from the well surface to a packer set at a preselected location above a production zone. In the case of wells of substantial depth, and particularly wells where the downhole temperatures are substantially in excess of the surface temperatures, problems have been encountered due to excessive expansion or contraction of the elongated tubing string For example, in the treatment or stimulation of the well, it is common to introduce fluids at surface ambient temperature into the tubing string. When the major portions of the tubing string are at a much higher temperature, this inherently results in a cooling, and hence a substantial contraction of the tubing string, resulting in the production of a substantial tensile stress in the tubing string between its surface connection and the set packer. Similarly, in the production phase of such wells, the production fluid is normally at a temperature substantially in excess of the temperature of the majority of the tubing string, resulting in a substantial expansion of the tubing string and the production of a substantial compressive force on the tubing string.
Additionally, changes in fluid pressure inside and outside the tubing string play a major role in the development of substantial tension or compressive forces in the tubing string.
The prior art has attempted to cope with this problem by incorporating expansion joints in the tubing string and generally located between the bottom of the tubing string and the packer. Such expansion joint may, for example, comprise an elongated seal bore receptacle attached to the packer within which there is sealingly telescopically mounted a mandrel connected at its upper end to the tubing string and relatively movable with respect to the seal bore of the receptacle in response to the changes in tension or compression in the tubing string. Such relatively movable telescoped elements require a plurality of dynamic seals disposed therebetween to maintain the sealing integrity of the tubing string. As a result, such seals are continuously subject to axial motions and, as is wellknown in the art, movable seals tend to deteriorate more rapidly than the non-movable seals under the adverse environmental conditions encountered in subterranean wells. To overcome this problem, the prior art practice has been to land the tubing string with a sufficient amount of compression to prevent dynamic movement of the expansion joint during all cycles except treatment or stimulation; however, this technique results in the development of excessive compressive forces during production modes, thus threatening the structural integrity of the tubing string.
It would therefore be desirable to provide an expansion joint which did not permit movement of the seal elements until the tension in the tubing string approaches the level where fracture of the tubing string might be encountered, as during treatment or stimulation of the well. Following the treatment or stimulation, the flowing of the well would result in an expansion of the compressed tubing string and the mandrel of the expansion joint would move back to its original contracted position relative to the seal bore receptacle where it would be again latched so as to prevent relative movement of the seals along the seal bore until an extreme tensile stress condition was again encountered.